1. Field of the Invention
The invention relates to a molding machine, more particularly to an extrusion die device for a hollow molding machine.
2. Description of the Related Art
A hollow molding machine generally comprises a feeding device, an extrusion die device connected to the feeding device, and a molding device registered with the extrusion die device. In use, solid plastic is melted in the feeding device and fed into the extrusion die device. Afterward, the melted plastic is extruded from the extrusion die device into the molding device, and air is blown into the molding device to inflate the plastic into a desired hollow shape. Some hollow molding machines even have multiple feeding devices connected to one extrusion die device for feeding plastics with different colors into the extrusion die device such that a multicolor plastic can be extruded.
As shown in FIG. 1, a conventional extrusion die device 1 for a hollow molding machine comprises a base 11, three extruding units 12, and a control unit 13. The base 11 includes three concentric annular passages 111, three feed inlets 112 extending radially, outwardly, and respectively from the passages 111, and a die exit 113 that is in spatial communication with the bottom ends of the passages 111. Each of the feed inlets 112 fluidly communicates with a feeding device 10 of the molding machine. Each of the extruding units 12 includes an annular plunger 121 disposed movably in a respective one of the passages 111, and a hydraulic actuator 122 for driving upward and downward movements of the plunger 121. The control unit 13 includes a plug 131 extending movably through the plunger 121 of the innermost one of the extruding units 12. When the molding machine is in use, the plug 131 is driven to move upwardly such that a plug head formed at a distal end of the plug 131 closes the die exit 113. At that time, melted plastics are fed respectively by the feeding devices 10 into the passages 111 via the feed inlets 112, and are retained in the die exit 113. Afterward, the plug 131 is driven to move downwardly along with the plug head so as to open the die exit 113, and the plunger 121 of each of the extruding units 12 is driven immediately by the corresponding hydraulic actuator 122 to move downwardly, thereby extruding the melted plastics outwardly of the die exit 113 into a molding device (not shown) of the molding machine for subsequent processing.
In case the hydraulic actuators 122 are not actuated simultaneously, the slower one will have to overcome a larger pressure to drive movement of the corresponding plunger 121. Therefore, the extruding unit 12 further includes a servo motor (not shown) to ensure that the hydraulic actuators 122 will operate simultaneously. However, the installment of the servo motor results in a relatively complicated structure of the conventional extrusion die device and a relatively high manufacturing cost.